Method of forming composite aluminum-steel parts by casting aluminum onto steel andbonding thereto



Patented Mar. 13, .1951

U NIT-ED STATES PATENT OFFICE A METHOD OFFOR-MING COMPQSITE ALU-RHNUM-STEEL PARTS BY CASTING ALUMWUM ON-T STEEL AND BOND- 1 .IN G'IHERETO Howard Grange and Dean K. Harlin-k, Detroit,

Mich, assig-nors to General Motors Corporation, Detroit, Mich, acorporation of Delaware No Drawing.

IO-Claims. 1

This invention relates to forming composite articles by casting andbonding aluminum or an aluminum base alloy onto steel or other ferrousmetal.

The primary object of our invention is to provide a simple and practicalprocedure for strongly bonding aluminum or aluminum base alloys to steelor other ferrous metal parts. The simple procedure results in forming astrong bond between steel and aluminum that has a relatively low degreeof brittleness at the junction between the two metals.

In carryingout the invention the steel or other ferrous metal part isimmersed in a molten or fused salt bath capable of absorbing iron oxideand aluminum oxide. A salt bath of the following composition has provenhighly satisfactory.

47% Potassium Chloride (K01) 35% Sodium Chloride (NaCl) 12% Cryolite(NazAlFs) 6% Aluminum Fluoride (AlFs This salt bath has a melting pointof about 1180 F. Addition of a small amount of lithium chloride to theabove composition lowers the 'melting'point thereof. For example, aboutlithium chloride lowers the melting point to about 1075 F.

- The exact composition of the salt bath is not critical and theproportions of potassium chloride, sodium chloride, cryolite andaluminum Jfluoride may be varied from the above. For example, thepotassium chloride may range from about 37 to 57%, the sodium chloridefrom about to 45%, the :cryolite from about .8 to 20% and thealuminumfluoride from about 0.5 to 12%. The bath composition usually preferredis one that .become molten when heated to 1200 F. or somewhat lower.While in the foregoing examples the double salt NasAlFs (cryolite) isgiven, it should be understood that an equivalent amount of thiscomponent may be supplied in the form of the single salts, sodiumfluoride and aluminum fluoride. We have found that it is essential toprovide an excess of AlFa over that of the .cryolite ratio in order toobtain the desired results.

"I'he' temperature of the molten or fused salt bath should be maintainedabove 1250 F. to obtain effective fiuxing action. To avoid excessivevolatilizati'on and chemical instability of the molten salt it shouldnot be maintained higher than 1600 F. Salt bath temperatures within therange of 1300 to 1450 F. are preferred. The fused salt bath must beactivated by aluminum Application August 12, 1947, Serial No. 7 68,270

in or in contact with the fused salt in order to provide effectivefluxing action. This may be done byemployi-ngan aluminum or aluminumalloy-coated container for the fused salt, or aluminum or aluminum alloycan be added to the salt. The aluminum or aluminum alloy may be added byimmersing a bar or sheet of the metalin the fused salt bath. The bar orsheet of aluminum or aluminum alloy soon melts and goes to the bottom ofthe bath.

The steel or other ferrous metal part may be preheated, if desired,prior to immersion in the salt bath and this permits use of smallerquantities of salt and smaller sized salt bath heating means. Wherepreheating is employed it is prefer-red that the ferrous metalbe'preh'eated under conditions such that the surface is not oxidized.For this purpose heating in a non-oxidizing 'or reducing atmospherefurnace such as one employing hydrogen, Drycolene, etc., may be used.The term Drycolene is the trade-name for a furnace atmosphere gasproduced in a charcoal generator utilizing a hydrocarbon gas and air asa gas source The air and hydrocarbon gas are passed through hot charcoal"at 1800 F. and transformed by chemical reaction with the charcoal intoan atmosphere consisting of approximately/ 20% carbon monoxide, .lessthan 2% hydrogen, less than 5% carbon dioxide, and the balance nitrogen.The preheating temperature preferably is on'the order of .1200" F. to1600 F. Where the ferrous metal Ii'spreheated to the temperature of thesalt bath in a reducing atmosphere and is free of oxides of iron andother foreign matter, fthe'time of immersion may be as little as one ortwo seconds if no complicated recesses are present. If complicatedrecesses are present a longer time may be required to ensure that thesalt thoroughly cover or coat the ferrous metal part at those portionsthereof to which the aluminum or aluminum ibase alloy is to be bonded.Where the ferrous metal has oxides of iron or other foreign matter onthe surface thereof longer times of immersion will be required in orderto obtain clean surfaces. Where the preheating step is not used a longertime of immersion is required. The time should be at least sufficient toheat the ferrous metal part to a temperature of at least about l250 F.The exact time required will, of course, depend on the mass of the partand the size and thermal efiiciency of the furnace and salt containingvessel. Holding the ferrous metal in the fused salt for an extendedperiod of time is not harmful. The fused salt bath serves to clean thesteel or other ferrous metal from undesirable oxides and to flux themetal preparatory to dipping or immersing in a zinc alloy bath. Nospecial preparation of the surface of the ferrous metal part prior toimmersion in the fused salt bath is required since the molten salt bathflux dissolves iron oxides. The ferrous metal part, of course, must bedry to avoid a steam explosion when immersed in the molten salt. A smallamount of oil will burn off with no deleterious effect. Where theferrous metal part has an excessive amount of oil thereon or is badlyrusted, known oil removing and rust removing procedures may be employed.

After treatment in the fused salt bath the ferrous metal part is thenremoved and immersed in a zinc alloy bath. A zinc alloy bath composed ofabout 9% aluminum and the balance zinc has proven highly satisfactory inpractice. However, other zinc base alloy baths containing from about .5%to 12% aluminum also have been used successfully. A small or minuteamount of aluminum must be present to protect the zinc from excessiveoxidization. Aluminum, if present in the zinc-aluminum alloy in excessof about 2%, results in formation of an excessive quantity of a complexcompound of iron, zinc and aluminum that adheres to the steel or otherferrous metal part in a thick viscous deposit that interferes withaccurately locating the part within the mold cavity. The temperature ofthe zinc alloy bath is within the approximate range of 1300 F. to 1500F. Temperatures of 1325" F. to 1375" F. are preferred. The time ofholding the steel or other ferrous metal part in the zinc alloy bath canbe as little as two seconds since a zinc alloy coating forms readily onthe ferrous metal part. An extended period such as ten minutes ispermissible and may be desirable in some cases to permit displacement ofmolten salt from parts of complicated recessed design.

The zinc alloy coated ferrous metal part is then removed from the zincalloy bath and inserted in located position in a mold, the mold closed,and

aluminum or aluminum base alloy poured into the mold while the zincalloy coating is still molten or mushy. The mold gates preferably are sodesigned as to direct the incoming molten aluminum or aluminum-basealloy up and around the zinc-coated ferrous metal part. The coatedferrous metal part should be at or near the temperature of the aluminumor aluminum base alloy to avoid chill and blow effects and to promoteremoval of excess zinc alloy and final alloying of the aluminum oraluminum base alloy with the coated ferrous metal. After cooling, theferrous metal and the cast aluminum or aluminum base alloy are stronglybonded into an integral assembly.

By aluminum base alloys is meant those aluminum alloys which containabout 80% or more of aluminum.

In one application of the invention the process has been employed informing gear blanks consisting of a steel hub to which is bonded analuminum alloy web and flange portion. In this instance the steel hubwas formed of G. M. 1112 steel and Alcoa #142 aluminum casting alloy wasused. The aluminum alloy of these gear blanks may be precipitationtreated at 350 F. for hours with a resulting hardness of 94 to 100Brinell Hardness number with 500 kg. load.

Various changes and modifications of the embodiments of our inventiondescribed herein may be made by those skilled in the art withoutdeparting from the principles and spirit of our invention. I

We claim:

1. The method of forming a composite metal product which comprisesimmersing ferrous metal in a fused salt bath composed about as follows:37% to 57% KCl, 25% to 45% NaCl, 8% to 20% NasAlFa, 0.5% to 12% AlFs,said fused salt bath having aluminum in contact therewith, said fusedsalt bath being at a temperature of about 1250 F. to about 1600 F., saidferrous metal having a temperature while in said molten salt bath of atleast about 1250 F., removing the heated ferrous metal from the moltensalt bath and while it is still heated immersing the same in a moltenzinc alloy coating bath composed predominately of zinc and containingabout .5% to 12% aluminum, said Zinc alloy coating bath being maintainedat a temperature of about 1300 F. to 1500 F., holding the ferrous metalin said zinc alloy bath for at least about two seconds to form a coatingthereon, removing the coated ferrous metal part from said molten zincalloy coating bath, and, before the coating has completely solidified,applying a molten metal consisting of at least about aluminum to saidcoated fer rous metal. I

2. A method as in claim 1 in which the ferrous metal is preheated undernon-oxidizing conditions prior to immersion in the fused salt bath.

3. A method as in claim 1 in which the ferrous metal is immersed in thefused salt bath while the ferrous metal is at room temperature and isheated in said fused salt bath to a temperature of at least 1250 F. a

4. A method as in claim 1 in which the zincalloy is composed of about 9%aluminum and the balance substantially all zinc.

5. A method as in claim 1 in which a small proportion not greater than20% of lithium chloride is added to the fused salt bath.

6. A method as in claim 1 in which the fused salt bath is operated at atemperature within the range of mom-14cc F.

'1. A method as in claim 1 in which the .zinc alloy coating bath ismaintained at a temperature within the range of 1325 to 1375" F.

8. The method of forming composite metal articles which comprisesimmersing steel in a fused salt bath composed approximately as fol lows:

47% potassium chloride 35% sodium chloride 12% cryolite 6% aluminumfluoride said fused salt bath having aluminum in contact therewith, saidfused salt bath being maintained at a temperature within the range of1300 to 1450 F., holding the steel in said molten salt bath until thesteel reaches a temperature of at least 1250 F., removing the heatedsteel from the molten salt bath and while the steel is still heatedimmersing the same in a zinc alloy coating bath composed of 9% aluminumand the balance su b' stantially all zinc, said zinc alloy coating bathbeing maintained at a temperature withinthe range 1325 F. to 1375 F.,holding the steel in said zinc alloy coating bath for a time of at leasttwo seconds to form a coating on the steel, re-j moving the coated steelfrom said moltenzino alloy coating bath and before the coatinghascompletely solidified inserting the coated steel in a mold and castingmolten metal consisting of REFERENCES CITED The following references areof record in the file of this patent:

UNITED STATES PATENTS Number Name Date 597,111 Gooch Jan. 11, 1893Number Number 6 Name Date Dean May 8, 1923 Post Aug. 18, 1925 DeputyJune 2, 1931 Bonath et al. Jan. 5, 1937 Mattsson Dec. 7, 1937 RohrigMar. 29, 1938 Deputy July 12, 1938 Wille et a1 Feb. 28, 1939 McCulloughet a1. Dec. 9, 1941 FOREIGN PATENTS Country Date Great Britain Nov. 17,1927

8. THE METHOD OF FORMING COMPOSITE METAL ARTICLES WHICH COMPRISESIMMERSING STEEL IN A FUSED SALT BATH COMPOSED APPROXIMATELY AS FOLLOW: